In general the invention concerns a new class of optoelectronic devices which can store information and/or perform logic functions by means of an optical memory substance which is contained in individually addressable cells or elements. Each element is an independent unit and can be combined with similar elements to form larger assemblies, typically in the form of planar sheets or layers. The latter may be configured in tertiary structures, for example by stacking in order to form optical data memories and optical logic devices with a high performance-to-volume ratio.
Present-day digital optical data storage technologies has been developed in response to an ever-expanding need for data storage capacity in a compact format and they have been remarkably successful in providing solutions which combine a high areal data density with replaceability and/or portability.
The crucial step has been the use of small, efficient semiconductor lasers emitting coherent light which can be focused to near-diffraction-limited spots, thus providing a correspondingly accurate definition and dense arrangement of the information bits in the data carrying medium. In practical systems, cost and space limitations have logically resulted in a design in which the laser beam is scanned across the surface of a rotating disc, picking up a serial bit stream as it follows an optical guidance track under servo control.
Systems based on this generic design have now been refined to a point where the data density is close to the theoretical limit, and further improvements in order to match future demands cannot be met by incremental improvements as in the past.
One obvious limitation is the use of two-dimensional data storage format. Even though the area data density is high, physical bit positions are restricted to a planar surface on a rigid, self-supporting, surface of high mechanical quality, leading to a relatively unimpressive volumetric data density. Technical solutions have recently been published wherein data are stored in several planes at different depths below the surface of the disk. Discrimination between different layers is possible due to the very short depth of field associated with a precise focus and this principle is expected to be developed to encompass up to ten planes or layers (see e.g. E. K.(signature), "Stacking the decks for optical data storage", Optics and Photonics News, August 1994, p. 39). It appears, however, that the benefits reaped from multiple layers or levels are partially neutralized by cost issues as well as technical trade-offs between the number of layers on the one hand and the achievable areal data density in each layer on the other hand. Even when implemented according to the claims set forth, such technological solutions lack the potential for sustained long-term development and improvement.
In many instances limitations in access time and data transfer rates represent a much more serious drawback for rotating disk systems than do the above-mentioned limitations on data densities and capacities.
In applications where files on a disk have to be accessed quickly in random sequence, the laser focusing servo must rapidly move an optical assembly radially from one position on the disk to another. At the correct radial position, tracking must be resumed quickly, involving alignment in two dimensions, adjustment of the spinning speed, establishment of synchronization and finding and identifying the file header. These electromechanical procedures involve access times which are long, typically 200 ms or more. Efforts have been made to reduce the access time, e.g. by increasing the disk rotation speed in order to reduce the time taken for rotational alignment, and reduce the weight of the servo-controlled focusing and tracking components. Improvement in one area, however, carries penalties in another. Increasing the spinning speed aggravates the so-called "whipsaw effect", i.e. the rapid acceleration and deceleration of rotation speed which is required in order to maintain a constant beam scanning speed over the surface of the disk when alternating between tracks at different radii. This is a dominant cause of latency in optical disc-based data retrieval systems. Attempts to eliminate the whipsaw effect by running at constant rotation speed irrespective of radial position lead to a reduction in the area data density or increased technical complexity. It is not surprising that such precision electromechanical optical systems will be slow on timescales which are typical in the purely electronic realm (microseconds or shorter), thus barring optical disk devices from use as direct rapid access memories in a wide range of applications, including as direct random access memories (DRAMs) for computers etc. Considerable efforts have been made to eliminate the Achilles heel of such devices, namely the need for focusing and tracking without mechanical inertia. Solutions which have been investigated include optoelectronic deflectors, waveguides and diffractive optical elements. So far no technical and financially viable systems of this nature have been demonstrated in practice and appear to lie many years in the future. In addition, the latency problem associated with disk rotation is not solved by such measures.
In rotating disksystems the data bits are read consecutively as the laser beam scans along the track, and the data transfer speed is explicitly dependent upon the rotation speed and the linear data density along the track. In a number of applications, such as interactive multimedia, the transfer speed is a significant bottleneck in present-day optical disk systems. Given the near- optimal encoding and focusing of data typical of present-day developments in disktechnology, there seem to be few options available for increasing the data transfer speed. One possibility is to increase the rotation speed. This has been done in several commercial systems to a point where cost and power consumption now lead to a rapidly diminishing return on further speed increases. Another strategy is to employ several laser beams addressing separate, but parallel tracks on the disk. As the number of parallel tracks increases, however, the complexity and cost increase very rapidly, and such schemes seem at best to be destined to provide speed improvements which fall far short of projected future needs.
The above shortcomings have been clearly recognized for a long time, and other schemes have been proposed and experimentally investigated, most notably page-oriented memory and logic systems, based on holographic technology. In addition to promising high density volumetric data storage in three dimensions, holographic systems can be addressed in a page-mode version, thereby offering the advantages which are inherent in parallelism, such as a high transfer speed. Rapid random accessing of data by means of inertia-free optoelectronic means is under investigation. Furthermore, logic operations have been investigated such as high speed parallel processing for object recognition. It has been predicted that holographic memories can be erased and rewritten repeatedly, a data volume at giga- to terabyte level can be stored in a volume comparable to a sugar cube, giving random access times in the micro- to nanosecond range and data transfer speeds of hundreds of Mbyte/s (see e.g. D. Psaltis and F. Mok, "Holographic memories", Scientific American, November 1995, pp. 52-58). Similar potential performances have been mentioned for other systems based on confocal and multilaser (non-linear) addressing principles (see e.g. "The optical sugar cube", Photonics Spectra, September 1994, p. 50).
A further example of a page-based optical data storage system which may be mentioned is internationally published patent application no. W096/2 1228 (Birge) entitled "Branched photocycle optical memory device", which discloses a volumetric optical memory which stores information at high density in three dimensions by selectively activating a photochemical branch reaction from a transitory thermal intermediate state in the primary photocycle in a light-sensitive protein-based storage medium. In this case a so-called "paging" laser is used to activate a planar layer or a page of the data storage medium on one wavelength and data lasers which transmit on another wavelength selected data beams which are orthogonal to the selected layer or page. However, this technology is not easy to implement in practical data storage devices and has some significant weaknesses. In order to obtain high volumetric data density the paging light must be extremely intense and uniform within a very narrowly defined spatial range with a sharp intensity limit. This entails the use of a laser beam and relatively complicated optics in order to form the beam. Secondly, a very precisely controlled illumination sequence is required, involving the use of three separate wavelengths. The optimal time control of the sequences is temperature-dependent. Thirdly, the write and read speeds are limited by the photocycle's time constants, resulting in access times in the ms range. Fourthly, reading of the stored data will reduce the contrast thereof on optical memory media, thus necessitating refreshing after a certain number, for example, 1000 read operations.
In SE patent no. 501 106 (Toth), entitled "Optical memory", there is disclosed an optical memory of the type Write-Once-Read-Many-Times (WORM type) which contains a storage element with stable optical states. The storage element is divided into a number of memory positions, the optical state in a given memory position being capable of being both changed and read out by means of a light beam directed towards the memory position. The memory can be realized entirely without movable mechanical parts and has a very short addressing time, permitting a particularly high storage capacity. This memory also permits parallel writing and reading of multibit words. The actual storage medium may be provided in several layers or levels. The light beam is then focused on a given memory position, and with the use of eight levels it becomes possible to store one byte in each memory position or x,y position. In a design with 7.7 storage cells of each 1 cm.sup.2, 9.8 Gbyte can be stored on eight levels and the write speed will then be 40 Mbyte/s. Read-out is performed in absorption mode, which means that the individual levels must have fixed, different thicknesses in order to make it possible to distinguish between the individual levels in the code sequence. This results, however, in a reduction in the volumetric storage density as the number of levels increases, and the necessity of focusing the light beam on a memory position as well as manoeuvring the light beam in the x,y directions entails cost-related and technical complications.
Even though the hitherto proposed technical solutions may appear impressive, in a future commercial environment such performances must be assessed with a view to hardware cost, system complexity and overall device form factor. Based on the current state of the art as revealed in the open literature, it seems correct to conclude that holographic and other page-oriented systems or multilayer systems will not achieve a breakthrough within the foreseeable future in markets where the demand for compactness and low cost is at a premium. Even if components and materials were to become available at acceptable costs, in reality the proposed architectures seem to preclude truly compact solutions.